This invention relates to methods for remote inspection of solid structures such as metals, and, in particular, to a method for remotely inspecting solid structures for discontinuities other than surface imperfections. Within the fields of manufacturing and quality control, there often exists a need to remotely inspect a surface for cracks or other discontinuities. In the past, optical techniques have primarily been of the direct detection type utilizing either visible or infrared light. The technique disclosed in the present invention utilizes heterodyne detection instead of direct optical detection. Direct detection has difficulty distinguishing a true material defect from a superficial imperfection such as rust, scale, or minor scratches. The present invention as hereinafter described has the advantage that it will only detect a true discontinuity in a structure and will not give false indications due to surface defects. Another useful application of the present invention is remote analysis of spot weld integrity sheet metal structures. At a point where the sheet metal is spot welded there will be a local change in vibration frequency due to the presence of the additional mass of the weld.
Optical and infrared heterodyne detection is described in articles such as by D. E. LeLange, "Optical Heterodyne Detection," IEEE Spectrum 5, 77 (1968) and by M. C. Teich, "Infrared Heterodyne Detection," Proc. IEEE 56, 37 (1968). An article by W. Puschert "Optical Detection of Amplitude and Phase of Mechanical Displacements in the Angstrom Range," Optic Communications, Vol. 10, No. 4, 357, April 1974, discusses a method for detecting light from a periodically vibrating surface.